1. Field of the Invention
The invention relates to a speed reduction mechanism that is suitably used in, for example, an electric vehicle that has an electric motor as a driving source and a motor torque transmission device that includes the speed reduction mechanism.
2. Description of Related Art
There is a conventional motor torque transmission device that is mounted in an automobile, and that includes an electric motor and a reduction-transmission mechanism (see, for example, Japanese Patent Application Publication No. 2007-218407 (JP 2007-218407 A)). The electric motor generates motor torque. The reduction-transmission mechanism reduces the speed of rotation output from the electric motor and transmits driving force to a differential mechanism.
The reduction-transmission mechanism of the motor torque transmission device of this type has a pair of disc-shaped revolving members formed of curved plates, a plurality of outer pins, and a plurality of inner pins. The revolving members each are an example of an external gear mechanism that revolves in accordance with the rotation of a motor shaft of the electric motor, the motor shaft having eccentric portions. The revolving members each have a plurality of wave-shaped portions at its outer peripheral portion. The wave-shaped portions are formed of a trochoidal curve, such as an epitrochoidal curve. The outer pins are an example of an internal gear mechanism that applies rotation force to the revolving members. The inner pins are arranged radially inward of the outer pins, and output the rotation force of the revolving members to the differential mechanism as driving force (torque), and the driving force is transmitted to a rotation member at wheel side.
The revolving members each have a center hole and a plurality of pin insertion holes that are arranged at equal intervals around the central axis of the center hole. The central axis of each center hole is offset from the axis of each eccentric portion of the motor shaft. The revolving members are rotatably supported by the respective eccentric portions of the motor shaft via bearings (cam bearings).
The outer pins are arranged around the axis of the motor shaft at equal intervals, and are connected to a housing of the reduction-transmission mechanism.
The inner pins are passed through the pin insertion holes of the revolving members. The inner pins are fitted to a differential case. Bearings (pin-side bearings) are fitted to the inner pins. The bearings are used to reduce contact resistance between the inner pins and the inner peripheries which define the pin insertion holes of the revolving members.
In the motor torque transmission device described in JP 2007-218407 A, a plurality of outer pins needs to be prepared, and further, the outer peripheral portions of the revolving members need to be formed into a complex shape, which is uneconomical.
To avoid such a problem, external gears, each of which is an example of an external gear mechanism, may be employed as revolving members, an internal gear, which is an example of an internal gear mechanism, may be employed as a rotation force applying member that applies rotation force to the revolving members. In addition, the number of teeth of the internal gear may be set larger than the number of teeth of each of the external gears.
However, when a reduction-transmission mechanism formed of an external gear mechanism and an internal gear mechanism, such as the reduction-transmission mechanism formed of the outer pins and the disc-shaped curved plates each having the wave-shaped portions formed of a trochoidal curve, such as an epitrochoidal curve, at its outer peripheral portion, as described in JP 2007-218407 A, or the reduction-transmission mechanism formed of the external gears and the internal gear, is used in a motor torque transmission device for an automobile, the revolving speed of the external gear mechanism that is a revolving member becomes relatively high. Therefore, a load due to centrifugal force acts on the cam bearings from the revolving members when the torque is output. As a result, it is necessary to use bearings with high durability as the cam-side bearings, resulting in a cost increase. In addition, because a load due to centrifugal force acts on the cam-side bearings, the service life of each of the cam-side bearings is shortened.